First of all. We need to determine if you have a high parasitic draw on your battery.
The best way to check this is to get a multimeter and put it inline on the ground side of your battery with everything turned off.
like this video shows.
Next. Im just guessing here but most likely he put the battery into something like a Vat 40 battery tester. Those are very precise and can measure a batteries real power. Voltage is meaningless in this situation its all about the CCAs cold cranking amps.
If you find you have high draw I would remove the trailer extension and retest. If it changes your results you know its the trailer wiring. If it doesnt change then most likely its your trucks wiring.
If it shows low draw then get your battery replaced and if your still having issues look into your charging system voltage regulator and alternator.
good luck!